Multiple Citations without outer parentheses

You can use natbib package by adding in your preamble:

\usepackage{natbib}

and then, according to natbib documentation when you use:

\citeauthor*{jon90} --> Jones, Baker, and Williams

\citet*{jon90} --> Jones, Baker, and Williams (1990)

\citep*{jon90} --> (Jones, Baker, and Williams, 1990)

\citealt{jon90} --> Jones, Baker, and Williams, 1990